Pressure swing adsorption is an important gas separation process which is widely used in the process and manufacturing industries. Pressure swing adsorption is used for recovering high-purity gas products from crude process gas streams, for example in hydrogen production, or as an alternative to hauled-in atmospheric gas products or onsite cryogenic air separation processes. The pressure swing adsorption process has been highly developed for the separation of a wide variety of gas mixtures including, for example, the separation of air to provide oxygen and nitrogen products.
Pressure swing adsorption processes can be operated wherein the maximum and minimum cycle pressures are both superatmospheric, wherein the maximum cycle pressure is superatmospheric and the minimum cycle pressure is subatmospheric, or wherein the maximum cycle pressure is near atmospheric and the minimum cycle pressure is subatmospheric. The latter two processes have been described in the art as vacuum-pressure swing adsorption (VPSA) and vacuum swing adsorption (VSA). For the purposes of the present disclosure, the generic term xe2x80x9cvacuum swing adsorptionxe2x80x9d or VSA will be used to describe any cyclic gas adsorption process which utilizes the effect of pressure on adsorbent capacity to separate gas mixtures wherein at least a portion of the adsorption cycle is operated at subatmospheric pressure.
Each adsorbent bed in a VSA cycle proceeds through a sequence of steps beginning with a feed or adsorption step in which a pressurized feed gas mixture is passed through a bed of adsorbent which selectively adsorbs one or more of the components in the mixed feed gas. A product gas containing the desired component at acceptable purity is withdrawn from the bed until the adsorption step is terminated at a predetermined time.
After termination of the adsorption step, the pressure in the bed is reduced in one or more steps in which gas is transferred at decreasing pressure to one or more other beds to provide pressurization gas to those beds. Final depressurization typically is completed by evacuation using a vacuum blower. The depressurized bed then is purged with product gas or transfer gas provided from other beds, thereby removing additional adsorbed components and void space gas from the bed.
Upon completion of the purge step, the bed is repressurized to an intermediate pressure by one or more pressurization steps in which gas is transferred from other beds, and the bed then is pressurized further to the feed pressure with feed and/or product gas. The steps are repeated in a cyclic manner.
The transfer of gas from a bed at decreasing pressure to another bed at increasing pressure is a useful feature of many VSA cycles. In this bed-to-bed gas transfer process, gas that is below product quality, but that still contains a significant concentration of the final product component, is transferred from the product end of a bed to the product end of another bed. This step may increase product recovery, but must be carefully controlled to meet the required product purity.
Further refinement in the bed-to-bed gas transfer process holds promise for needed improvements in product recovery and product purity, and also for increased productivity, in the VSA process. In particular, there is a need for improved control of gas flow within a bed undergoing gas withdrawal during the gas transfer process. This need is addressed by the present invention as described below and defined by the claims which follow.
The invention relates to a gas transfer segment of a vacuum swing adsorption process cycle utilizing multiple parallel adsorbent beds which undergo cyclic process steps to separate the components of a feed gas mixture, each bed having a feed end and a product end, wherein the gas transfer segment comprises a step of withdrawing a waste gas stream from the feed end of a first bed, withdrawing a transfer gas from the product end of the first bed and introducing the transfer gas into the product end of a second bed, and withdrawing a waste gas stream from the feed end of the second bed.
The gas transfer segment also comprises terminating the withdrawing of the waste gas stream from the feed end of the second bed while continuing to withdraw the waste gas stream from the feed end of the first bed, continuing to withdraw the transfer gas from the product end of the first bed, and continuing to introduce the transfer gas into the product end of the second bed.
The gas transfer segment may further comprise introducing the feed gas mixture into the feed end of the second bed while continuing to withdraw the waste gas stream from the feed end of the first bed, continuing to withdraw the transfer gas from the product end of the first bed, and continuing to introduce the transfer gas into the product end of the second bed.
The feed gas mixture may be air, the pressure in the second bed may be below atmospheric pressure, and the feed gas mixture may be introduced into the feed end of the second bed by atmospheric air flowing into the feed end of the second bed. The waste gas streams may be enriched in nitrogen relative to air.
The invention includes a vacuum swing adsorption process for recovering a less strongly adsorbable component from a feed gas mixture containing at least one less strongly adsorbable component and at least one more strongly adsorbable component, which process comprises performing cyclic process steps in a plurality of adsorbent beds, each bed having a feed end and a product end and containing adsorbent material which selectively adsorbs the more strongly adsorbable component, each bed proceeding in turn through cyclic process segments which include an adsorption-make product segment, a decreasing pressure gas transfer segment in which gas flows from a bed at higher pressure into one or more other beds at lower pressure or pressures, a regeneration segment, an increasing pressure gas transfer segment in which gas flows into a bed at lower pressure from one or more other beds at higher pressure or pressures, and a final repressurization segment, wherein the decreasing pressure gas transfer segment includes a step of withdrawing a first waste gas stream from the feed end of a first bed, withdrawing a transfer gas from the product end of the first bed and introducing the transfer gas into the product end of a second bed, and withdrawing a second waste gas stream from the feed end of the second bed.
The process also comprises terminating the withdrawing of the second waste gas stream from the feed end of the second bed, withdrawing a third waste gas stream from the feed end of the first bed, continuing to withdraw the transfer gas from the product end of the first bed, and continuing to introduce the transfer gas into the product end of the second bed. The process may further comprise introducing the feed gas mixture into the feed end of the second bed, withdrawing a fourth waste gas stream from the feed end of the first bed, continuing to withdraw the transfer gas from the product end of the first bed, and continuing to introduce the transfer gas into the product end of the second bed. The ratio of the quantity of the first waste gas stream to the total quantity of the first, third, and fourth waste gas streams may be between about 0.15 and about 0.30.
The feed gas mixture may be air and the pressure in the second bed may be below atmospheric pressure so that atmospheric air flows into the feed end of the second bed. The waste gas streams may be enriched in nitrogen relative to air. The first waste gas stream may be withdrawn by discharging from a superatmospheric pressure in the first bed directly to the atmosphere, and the third waste gas stream may be withdrawn from the first bed by a vacuum blower.
The invention includes a pressure swing adsorption process for recovering a less strongly adsorbable component from a pressurized feed gas containing at least one less strongly adsorbable component and at least one more strongly adsorbable component, which process comprises performing cyclic process steps in two parallel adsorbers, each adsorber having a feed end and a product end and containing adsorbent material which selectively adsorbs the more strongly adsorbable component, which cyclic process steps include:
(a) providing a feed gas at superatmospheric pressure and introducing the feed gas into the feed end of a first adsorber, selectively adsorbing a portion of the more strongly adsorbable component on the adsorptive material, and withdrawing from the product end of the first adsorber a product gas enriched in the less strongly adsorbable component;
(b) terminating the introduction of the feed gas into the feed end of the first adsorber, depressurizing the first adsorber by withdrawing a first waste gas stream from the feed end of the first adsorber and by withdrawing a transfer gas from the product end of the first adsorber, introducing the transfer gas into the product end of a second adsorber, and withdrawing a second waste gas stream from the feed end of the second adsorber;
(c) terminating withdrawal of the transfer gas from the product end of the first adsorber and withdrawing a third waste gas from the feed end of the first adsorber;
(d) pressurizing the first adsorber by any combination of (1) introducing feed gas into the feed end thereof; (2) introducing product gas into the product end thereof, and (3) introducing feed gas into the feed end thereof and introducing product gas into product the product end thereof; and
(e) repeating steps (a) through (d) in a cyclic manner.
The feed gas may be air, the at least one less strongly adsorbable component may be oxygen and the at least one more strongly adsorbable component may be nitrogen, the first waste gas stream from the first adsorber may be discharged directly to the atmosphere, and the third waste gas stream may be withdrawn from the first adsorber by evacuation with a vacuum blower.
The process may further comprise, following step (a), the additional step of withdrawing a portion of the product gas from the first adsorber and using this portion of gas to purge the second adsorber.
The process may further comprise, following step (b), the additional step of(b1) terminating the withdrawing of the second waste gas stream from the feed end of the second adsorber, withdrawing a fourth waste gas stream from the feed end of the first adsorber, continuing to withdraw the transfer gas from the product end of the first adsorber, and continuing to introduce the transfer gas into the product end of the second adsorber. The process may further comprise, following step (b1), the additional step of (b2) withdrawing a fifth waste gas stream from the feed end of the first adsorber, continuing to transfer gas from the product end of the first adsorber to the product end of the second adsorber, and introducing feed gas into the feed end of the second adsorber.
The feed gas may be air, the at least one less strongly adsorbable component may be oxygen and the at least one more strongly adsorbable component may be nitrogen. The first waste gas stream from the first adsorber may be discharged directly to the atmosphere, and the third, fourth, and fifth waste gas streams may be withdrawn from the first adsorber by evacuation with a vacuum blower. When the feed gas mixture is air, the pressure in the second adsorber may be below atmospheric pressure, and the feed gas mixture may be introduced into the feed end of the second adsorber by atmospheric air flowing into the feed end of the second adsorber.